Quality control process for UMG-SI feedstock

ABSTRACT

A quality control process for determining the concentrations of boron and phosphorous in a UMG-Si feedstock batch is provided. A silicon test ingot is formed by the directional solidification of molten UMG-Si from a UMG-Si feedstock batch. The resistivity of the silicon test ingot is measured from top to bottom. Then, the resistivity profile of the silicon test ingot is mapped. From the resistivity profile of the silicon test ingot, the concentrations of boron and phosphorous of the UMG-Si silicon feedstock batch are calculated. Additionally, multiple test ingots may be grown simultaneously, with each test ingot corresponding to a UMG-Si feedstock batch, in a multi-crucible crystal grower.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation in part ofpending U.S. patent application Ser. No. 12/703,727 “PROCESS CONTROL FORUMG-Si MATERIAL PURIFICATION” by Kamel Ounadjela and filed on Feb. 10,2010, which is incorporated herein by reference in its entirety and madepart of the present U.S. Utility patent application for all purposes.

This application claims the benefit of provisional patent application61/173,853 filed on Apr. 29, 2009, which is hereby incorporated byreference in its entirety.

FIELD

This invention relates in general to the field of silicon processing,and more specifically to the purification of upgradedmetallurgical-grade silicon.

BACKGROUND OF THE INVENTION

The photovoltaic industry (PV) industry is growing rapidly and isresponsible for an increasing amount of silicon being consumed beyondthe more traditional uses as integrated circuit (IC) applications.Today, the silicon needs of the solar cell industry are starting tocompete with the silicon needs of the IC industry. With presentmanufacturing technologies, both integrated circuit (IC) and solar cellindustries require a refined, purified, silicon feedstock as a startingmaterial.

Materials alternatives for solar cells range from single-crystal,electronic-grade (EG) silicon to relatively dirty, metallurgical-grade(MG) silicon. EG silicon yields solar cells having efficiencies close tothe theoretical limit, but at a prohibitive price. On the other hand, MGsilicon typically fails to produce working solar cells. Early solarcells using polycrystalline silicon achieved very low efficiencies ofapproximately 6%. In this context, efficiency is a measure of thefraction of the energy incident upon the cell to that collected andconverted into electric current. However, there may be othersemiconductor materials that could be useful for solar cell fabrication.In practice, however, nearly 90% of commercial solar cells are made ofcrystalline silicon.

Cells commercially available today at 24% efficiencies are made possibleby higher purity materials and improved processing techniques. Theseengineering advances have helped the industry approach the theoreticallimit for single junction silicon solar cell efficiencies of 31%.

Because of the high cost and complex processing requirements ofobtaining and using highly pure silicon feedstock and the competingdemand from the IC industry, silicon needs usable for solar cells arenot likely to be satisfied by either EG, MG, or other silicon producersusing known processing techniques. As long as this unsatisfactorysituation persists, economical solar cells for large-scale electricalenergy production may not be attainable.

Several factors determine the quality of raw silicon material that maybe useful for solar cell fabrication. Silicon feedstock quality oftenfluctuates depending on the amount of impurities present in thematerial. The main elements to be controlled and removed to improvesilicon feedstock quality are boron (B), phosphorous (P), and aluminum(Al) because they significantly affect the resistivity of the silicon.Feedstock silicon materials based on upgraded metallurgical (UM) siliconvery often contain similar amounts of boron and phosphorous. And whilechemical analysis may be used to determine the concentrations of certainelements, this approach requires too small of a sample size (a fewgrams) and often provides variable results—for example, the amount ofboron present may vary from 0.5 parts per million by weight (ppmw) to 1ppmw. Further, chemical analysis on different batches have providedconsistent boron and phosphorous concentrations but with extremevariation in electrical parameters. These unreliable results may be dueto the large affects relatively minor impurities produce.

Resistivity is one of the most important properties of silicon (Si) usedfor manufacturing solar cells. This is because solar cell efficiencysensitively depends on the resistivity. State-of-the-art solar celltechnologies typically require resistivity values ranging between 0.5Ω·cm and 5.0 Ω·cm. Currently produced feedstock materials based on UMsilicon often come with a base resistivity below the minimum resistivityof 0.5 Ω·cm that is typically specified by solar cell manufacturers.There is a simple reason for this: Expensive processes for upgradingUM-Si are primarily concerned with taking out non-metals, includingdopant atoms B and P. In order to reduce cost, there is a clear tendencyto minimize such processing, i.e., UM-Si typically still contains highconcentrations of dopant atoms.

Purification by segregation during directional solidification is oftenused in the process to obtain upgraded metallurgical silicon. Impurityremoval methods include directional solidification which concentratesimpurities such as B, P, Al, C, and transition metals in the last partof the resulting silicon ingot to crystallize—often the top of theingot. In a perfect case, the crystallization during the directionalsolidification process would be uniform from top to bottom and thesolid-liquid interface would be planar throughout the entire ingot. Thiswould result in consistent impurity concentrations profiles from top tobottom throughout the ingot—allowing impurities in the ingot to beremoved according to one flat cut across the ingot which removes toppart of the ingot.

However, controlling the thermal field during a directionalsolidification process is difficult and often results in aninhomogeneous growth of the crystals in the silicon ingot. This causesuneven top to bottom impurity concentration profiles throughout theingot (i.e. from one end of the ingot to another). This effect isfurther amplified in mass production of large amounts of silicon.Because different areas of the ingot have different impurity profiles,and thus different resistivity profiles, a flat cut across the ingotdoes not maximize the usable silicon yield while still removing most ofthe concentrated impurities.

Further, variability in incoming UMG-Si feedstock quality necessitatescontrol process for testing and analyzing UMG-Si material quality.Typically, elements such as boron (B) and phosphorous (P) can degrade Sifeedstock quality. If not controlled within certain concentration limitsthey produce sizable variations in ingot resistivity. Other elementssuch as, but not limited to, carbon, oxygen, nitrogen, compounds havingthese elements, in particular SiC, may also degrade ingot quality.

Due to the large effects of these and similar impurities, feedstockmaterials should be analyzed and tested to ensure proper quality. Batchto batch variations in the impurities and resistivity of incomingfeedstock affect bottom to top resistivity of ingots and yield (n-typepart vs. p-type part).

Suppliers of UMG-Si feedstock may not rigorously establish qualitycontrol of the materials they ship to their customers. Often, typicalchemical analyzes produces unreliable results because of the largeeffects relatively minor impurities produce. Further, suppliers oftentest too small a sample size in relation to the variability of boron andphosphorous concentrations in the feedstock batch. Additionally,superimposed measurement error makes measurement results uncertain. Oneindication of these measurement errors occurs when chemical analysis ondifferent batches yields the same boron and phosphorous content despitevariation in electrical parameters. For companies relying on a pluralityof UMG-Si feedstock batches to cast silicon ingots, these variationsamong batches may not be acceptable.

SUMMARY

Therefore a need has arisen for a quality control process for UMG-Sifeedstock material which provides reliable impurity data/measurements.The method must be accurate and provide impurity data for a feedstockbatch from a sample test ingot. A further need exists to more accuratelyidentify impurity concentration profiles in a UMG-Si feedstock materialbatch so that providers may more reliably produce UMG-Si meeting desiredimpurity concentration thresholds and solar cells manufacturers mayimprove silicon wafer yield.

A further need exists for a simple process that determines impurityconcentrations for UMG-based multi-crystalline silicon materialresulting in a material with good ingot yield and improved mechanicaland electrical properties, the latter in regard to solar cell quality.Such a process should be easily transferable to higher-grade, non-UMGfeedstock silicon which is used partially or exclusively forcrystallizing mono-crystalline silicon materials, for example byapplying the CZ technique or the FZ technique.

In accordance with the disclosed subject matter, a method fordetermining the concentrations of boron and phosphorous in a batchUMG-Si feedstock is provided that substantially eliminates or reducesdisadvantages and problems associated with previously developed UMG-Siimpurity concentration determination methods.

The present disclosure provides a method for determining theconcentrations of boron and phosphorous in a batch UMG-Si feedstock. Asilicon test ingot is formed by the directional solidification of moltenUMG-Si from a UMG-Si feedstock batch. The resistivity of the silicontest ingot is measured from top to bottom. Then, the resistivity profileof the silicon test ingot is mapped. From the resistivity profile of thesilicon test ingot, the concentrations of boron and phosphorous of theUMG-Si silicon feedstock batch are calculated.

According to one aspect of the disclosed subject matter, a plurality ofsilicon test ingots are grown simultaneously from different batches ofUMG-Si feedstock.

Technical advantages of the present disclosure include more accuratedata on silicon impurity concentrations which allows for obtaininghigher usable silicon yield, UMG-Si process control improvements, andUMG-Si manufacturing efficiency and cost improvements. A furthertechnical advantage of calculating the impurity concentrations of theUMG-Si feedstock batch based on the test ingot's resistivity profileincludes more consistent and accurate impurity concentrationmeasurements.

The disclosed subject matter, as well as additional novel features, willbe apparent from the description provided herein. The intent of thissummary is not to be a comprehensive description of the claimed subjectmatter, but rather to provide a short overview of some of the subjectmatter's functionality. Other systems, methods, features and advantageshere provided will become apparent to one with skill in the art uponexamination of the following FIGS. and detailed description. It isintended that all such additional systems, methods, features andadvantages included within this description, be within the scope of theaccompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

For a more complete understanding of the disclosed subject matter andadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numerals indicate like features and wherein:

FIG. 1 (Prior Art) is a process flow for reducing boron, phosphorous,and aluminum content in silicon;

FIG. 2 is graph showing actual measured impurities of various batches ofUMG feedstock;

FIG. 3 is a graph showing the concentration profile of impurities boronand phosphorous in a UMG-Si ingot;

FIG. 4 is a graph showing the resistivity profile (calculated vs.measured resistivity) of the UMG-Si ingot measured in FIG. 3;

FIG. 5 shows a cross-sectional image of a UMG-Si ingot after directionalsolidification;

FIG. 6 is a cross-sectional image of a UMG-Si ingot after directionalsolidification with cropping lines created in accordance with thedisclosed subject matter;

FIG. 7 is a graphical depiction of a 3-D solidification interface of asilicon ingot;

FIG. 8 is a graph showing the concentration profile of impurities boron,phosphorous, and aluminum in a UMG-Si ingot;

FIG. 9 is a cross sectional image of the aluminum concentration of theUMG-Si ingot depicted in FIG. 8;

FIG. 10 is a cross sectional image of the phosphorous concentration ofthe UMG-Si ingot depicted in FIG. 8;

FIG. 11 is a cross sectional image of the boron concentration of theUMG-Si ingot depicted in FIG. 8;

FIG. 12 is a process flow showing a side view of the solidification ofsilicon material in a dual directional solidification furnace;

FIG. 13 is a process flow showing a top view of the solidification ofsilicon material in a dual directional solidification furnace;

FIG. 14 is a graphical depiction of a 3-D solidification interface of asilicon ingot created in a dual directional solidification furnace;

FIG. 15 is a graph showing the resistivity profiles and cropping linesfor multiple impurity concentrations;

FIG. 16-18 are graphs illustrating the relationship between theresistivity profile and the impurity concentration profile of a siliconingot;

FIG. 19 is a graph showing the resistivity profiles (in ohm-cm oversolidification fraction) of the silicon ingots in FIGS. 16-18;

FIG. 20 presents the corresponding impurity concentration profiles forthe resistivity profiles in FIG. 19;

FIG. 21 is a graphic depiction showing a prior art process flow forreducing boron, phosphorous, and aluminum;

FIGS. 22 and 23 are graphs showing actual measured resistivity ofvarious batches of UMG-Si feedstock;

FIG. 24 displays the ICPMS data of B and P for exemplary test ingots ofa simultaneous directional solidification run;

FIG. 25 is a graph showing the measured resistivity data for Batch 1 inFIG. 24;

FIG. 26 is a graph showing the measured resistivity data for Batch 2 inFIG. 24;

FIG. 27 is a graph showing the measured resistivity data for Batch 3 inFIG. 24;

FIG. 28 is a graph showing the measured resistivity data for Batch 4 inFIG. 24;

FIG. 29 is a photograph of a casted ingot;

FIG. 30 pictorially depicts embodiments of crystal grower crucibleconfigurations in accordance with the disclosed subject matter;

FIG. 31 is a photographic representation showing a modification a singlecrucible thermal configuration to four (4) crucibles per run thermalconfiguration;

FIG. 32 is a photographic example showing impurities found in actualUMG-Si ingots; and

FIG. 33 is a process flow showing major steps of one embodiment of thedisclosed UMG-Si control process.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The following description is not to be taken in a limiting sense, but ismade for the purpose of describing the general principles of the presentdisclosure. The scope of the present disclosure should be determinedwith reference to the claims. And although described with reference tothe purification of aluminum-rich UMG silicon, a person skilled in theart could apply the principles discussed herein to any upgradedmetallurgical-grade material.

Preferred embodiments of the disclosed subject matter are illustrated inthe FIGUREs, like numerals being used to refer to like and correspondingparts of the various drawings.

FIG. 1 shows a prior art process flow for reducing boron, phosphorous,and aluminum content in silicon. In step 2, pure raw materials, such asquartz and coal, are selected to produce MG-Si with low boron content.Step 4 then reduces the aluminum content further through MG-Sirefinement. Additionally, boron content could be reduced further, forexample in a furnace with an oxygen fuel burner—leading ultimately toUMG-Si. Then to further reduce impurities such as boron, phosphorous,and aluminum, the UMG-Si is often processed through directionalsolidification systems until the silicon feedstock is ready to bereleased, typically when the boron concentration has been reduced toless than a specified threshold concentration. In both First DSS Pass 6and Second DSS Pass 8, the part of the ingot having the highestconcentration of impurities is cut off (usually the top part) yieldingpurer silicon. First DSS Pass 8 may yield silicon having impurities of,for example, greater than the required 0.5 parts per million by weightand Second DSS Pass 10 may yield silicon having impurities less than therequired 0.5 parts per million by weight.

More effective impurity controls are necessary to provide purer siliconwhile minimizing waste. Resistivity measurements on the silicon ingotafter first DSS Pass 8 and before cropping to remove impurities wouldsubstantially improve the silicon yield. Likewise, resistivitymeasurements on the silicon ingot after second DSS Pass 10 and beforesecond cropping to remove impurities would substantially improve thesilicon yield of the final silicon product.

FIG. 2 is graph showing actual measured concentrations in parts permillion by weight of selected elements in various batches of UMGfeedstock. Note the large variance in the concentrations of the elementsacross different batches of feedstock. This variability is mainly causedby source materials, such as quartz and coal, of UMG-Si feedstock. Smallvariations in impurity concentration can significantly affect thevariability from batch to batch of the bottom to top resistivity of theingot as well as the ingot yield (n-part vs. p-part). Aluminum 40, boron42, and phosphorous 44 are the main elements to be controlled as theysignificantly affect the resistivity of the material.

FIG. 3 is a graph showing the concentration profile of dopants boron 50and phosphorous 52 (in atoms per centimeter cubed over solidifiedfraction) in a UMG-Si ingot. In FIG. 3, the initial concentration ofboron 50 is 0.48 parts per million by weight and the initialconcentration of phosphorous 52 is 1.5 parts per million by weight. Thevariation of the concentrations of boron and phosphorous along thesolidified fraction (or ingot height) reflects uneven segregation duringdirectional solidification caused by element-specific segregationbehavior. The uneven segregation of boron and phosphorous in the ingotcauses a change of the conductivity type—from p-type (boron, aluminum)to n-type (phosphorous)—at around 80% ingot height. This change inconductivity type is shown by B/P ratio 54 (shown as the absolute valueof the difference B-P on FIG. 3). The B/P ratio, such as B/P ratio 54,thus limits the yield of p-type material. In case of UMG feedstockmaterial with a relatively high aluminum concentration, aluminum canalso impact the yield by shifting respective resistivity profiles.

FIG. 4 is a graph showing the resistivity profile (calculatedresistivity 62 and measured resistivity 60) of the UMG-Si ingotpresented in FIG. 3. The resistivity is measured in ohm-centimeters andthe ingot height is measured as a bottom to top percentage (translatinginto the solidified fraction g). The resistivity is determined from thenet-doping of the material which is the absolute difference in theconcentrations of boron and phosphorous (shown as abs(B-P) 54 in FIG.3). Note that the resistivity profile reflects the change inconductivity type caused by the segregation characteristics of boron andphosphorous in the ingot at around 80% ingot height as in FIG. 3.

FIG. 5 shows a cross-sectional image of a UMG-Si ingot after directionalsolidification. Impurity line 70 mirrors the measured change inconductivity type in a typical ingot based on Al-rich UMG-Si feedstock.In this cross sectional image of the ingot, a strong variation of theingot yield line may be observed (as shown by impurity line70)—indicating an ingot yield close to 90% on left ingot side 72 and aningot yield close to 60% on right ingot side 74. The heavy yieldvariation across the ingot reflects inhomogeneous thermal conditionsacross the ingot during solidification, leading to inhomogeneoussegregation conditions for the dopant elements B, P and Al.

Directional solidification typically concentrates impurities at the topof the ingot—and the top layer having the most impurities is thenremoved leaving the purer bottom layer for further processing. As shownin FIG. 5, layer 78 has fewer impurities than layer 77. However, UMG-Siingots rarely have a flat, planar impurity profiles after directionalsolidification. Flat cropping line 76 shows a flat cutoff line thatwould be typically used to remove impurities concentrated in the top ofthe ingot. However, a flat cut does not take advantage of the uneven andnon-uniform distribution of the impurities in the material (shown byimpurity line 70), thus leading to inefficient and wasteful UMG-Siprocessing.

FIG. 6 shows a cross-sectional image of a UMG-Si ingot after directionalsolidification with cropping lines created in accordance with thedisclosed subject matter. Impurities such as boron, phosphorous, andaluminum are doping active in the silicon and affect resistivities ofthe ingot bricks. Resistivity measurements provide an accuratedetermination of where to remove the contaminated part of the ingot inorder to decrease the absolute concentration of dopants and metallicimpurities in the ingot as a whole.

The lowest concentration of impurities is found in cool zone 80 (thefirst area to solidify). The highest concentration of impurities isfound in hot zone 82 (the last area to solidify). The segregation ofimpurities is concentrated in the last part of the ingot to besolidified from the melting state during directional solidification.This causes the impurity profiles to be different from region to regionin the ingot. Note the different impurity levels in ingot brick 86 andingot brick 94. The ingot has been cut into bricks in order to controlthe impurity removal by customizing a crop line for each brick. Ingotbricks 86, 88, 90, 92, and 94 have been cut after directionalsolidification. Cut line 84 reflects the brick divisions on the image.

After the bricks have been cut, a resistivity profile of the ingot iscreated by measuring the resistivity of the ingot from bottom to top andmapping those calculations, on a graph or 3-D resistivity map. Theresistivity measuring for the ingot may also take place before the ingothas been cut into bricks. Further, the brick size may be customizedaccording to many factors including, but not limited to, the size of thesilicon ingot, the impurity concentration of the silicon ingot, the sizenecessary for to obtain an accurate resistivity profile, andmanufacturing efficiency requirements.

In FIG. 6, impurity line reflects the impurity concentrations in theingot at threshold requirement levels. Standard cut shows a flat cropline that attempts to balance impurity removal with silicon materialyield. Controlled cut shows a customized crop line for each brick basedon that brick's resistivity profile. The controlled cut line defines thecalculated cropping line for each individual brick based on that brick'sresistivity profile—thus only those parts containing concentratedimpurities are removed while preserving silicon material yield. Thisallows for the optimal removal of impurities without sacrificing usablesilicon. The cut is calculated by measuring the resistivities from thetop to bottom of each brick.

Without performing a controlled cut according to the disclosed process,a traditional standard cut can leave many impurities in the ingot, suchas in the ingot region of brick 94, making it necessary to performanother directional solidification to further purify the materialstemming from such an ingot.

FIG. 7 is a graphical depiction of a 3-D solidification interface of asilicon ingot. Because it is difficult to control solidification, thesolid-liquid interface during ingot crystallization is not planar andresults in inhomogeneous segregation layers, as shown in FIG. 7. Afterdirectional solidification, the impurities are concentrated in the topof the ingot. However, solidification layers 90, 92, and 94 aresignificantly uneven—meaning, the solidification layers are not planarbut rather vary vertically upward and downward in the ingot and havevarying thicknesses throughout the ingot. This causes the impurityprofiles from region to region in the ingot to be different, whichresults in an uneven silicon ingot impurity profile. The unevensolidification layers make it difficult to easily and efficiently removethe concentrated impurities without sacrificing high yield silicon orleaving too many impurities in the ingot.

FIG. 8 is a graph showing the concentration profile of dopants boron100, phosphorous 102, and aluminum 106 (in atoms per centimeter cubedover ingot height percentage translating into solidified fraction g) ina UMG material ingot. In FIG. 8, the initial concentration of boron is0.411 ppmw, the initial concentration of phosphorous is 1.3 ppmw, andthe initial concentration of aluminum is 23.08 ppmw. Due to thedifferent segregation coefficient of boron, phosphorous, and aluminumduring directional solidification there is a change of the conductivitytype at approximately 87% ingot height. This change is reflected by theabsolute concentration of boron and phosphorous plus the concentrationof aluminum, abs(B-P+Al), shown as 104 on FIG. 8 and which defines thelimit of p-type material yield.

FIG. 9 is a cross sectional image of the aluminum concentration profileof the UMG-Si ingot depicted in FIG. 8. Again, due to the difficulty ofcontrolling the thermal field during the directional solidificationprocess, the crystallization layers become uneven yielding unevenimpurity concentration profiles. The concentration of aluminum increasesat the top of the ingot and fluctuates throughout the cross section ofthe ingot as shown by impurity line 110. This makes it difficult toefficiently remove aluminum and other impurities across an entire ingot.

FIG. 10 is a cross sectional image of the phosphorous concentrationprofile of the UMG-Si ingot depicted in FIG. 8. The concentration ofphosphorous increases at the top of the ingot and fluctuates throughoutthe cross section of the ingot as shown by impurity line 112. Theconcentration of phosphorous is significantly higher in certain portionsof the ingot making it difficult to optimally remove the phosphorousimpurities with one flat crop line along the entirety of the ingot.

FIG. 11 is a cross sectional image of the boron concentration profile ofthe UMG-Si ingot depicted in FIG. 8. The concentration of boronincreases at the top of the ingot and fluctuates throughout the crosssection of the ingot as shown by impurity line 114. The concentration ofboron is significantly higher in certain portions of the ingot making itdifficult to optimally remove the phosphorous impurities with one flatcrop line along the entirety of the ingot.

FIG. 12 is a process flow showing a side view of the solidification ofsilicon material in a dual directional solidification furnace. A dualdirectional solidification furnace is a solidification furnacecomprising a top and side heater—often arranged with a heater warmingthe top of the ingot and multiple heaters warming the sides of theingot—which concentrates impurities at the top and one side of resultingsilicon ingot. The dual directional solidification system of FIG. 12utilizes top heater 122 and side heaters 120 and 124 to concentrateimpurities both at the top of the ingot approximate top heater 122 andat the ingot side where side heater 120 is positioned. The liquidsilicon contains concentrated impurities, and is also known as thecontaminated area. At furnace temperature 1500° C., the silicon isentirely liquid. In step 126, the furnace temperature is reduced to1450° C., and the molten silicon partly solidifies—a solidified layer ofsilicon forms at the bottom of the ingot below the silicon melt. Thesilicon proximate top heater 122 remains molten while the silicondistant from top heater 122 crystallizes and the impurities concentratein the molten silicon. During step 126, side heater 120 and side heater124 are set at a uniform temperature and a vertical gradient ofsolidified silicon forms while the horizontal solidification gradient ofthe silicon remains uniform.

In step 128, at furnace temperature 1420° C., the silicon is mostlycrystallized and only the areas proximate to top heater 122 and sideheater 120 are molten—the remaining silicon has crystallized. Sideheater 124 and top heater 122 have cooled which allows the siliconproximate side heater 124 and top heater 122 to crystallize and themolten silicon moves proximate side heater 120. Impurities areconcentrated in the remaining liquid silicon in the top corner of theingot proximate heated side heater 120. Thus the impurities areconcentrated in the molten area closest top heater 122 and side heater120. This is the area which will be removed to purify the fullycrystallized silicon ingot. The dual directional solidification furnacemay be equipped with five holes on the top, one in the center, and fourin the corners in order to control and measure the height of thesolidified silicon part (often using a simple quartz rod). In step 130,at furnace temperature 1400° C., side heater 120 is cooled and thesilicon ingot is entirely solidified. The impurities are concentrated inthe crystallized area closest to top heater 122 and side heater 120. Theingot is now ready to be divided into bricks and the impurities removed.The dual directional solidification furnace uses the hot-zone near theheaters to concentrate impurities for efficient removal after thesilicon has crystallized completely.

In process, a vertical silicon solidification gradient is created as themolten silicon in the ingot begins to solidify. As the silicon in thebottom of the ingot cools, it solidifies and impurities (boron,phosphorous, and aluminum) move into the remaining molten silicon.Before the solid/liquid interface reaches the region of overchangeconductivity type (usually in the range of 80% ingot solidification) theside heaters adjust temperature to create a horizontal siliconsolidification gradient which directs the remaining molten silicon toone side of the ingot—the side proximate the hotter side heater.

FIG. 13 is a process flow showing a top view of the solidification ofsilicon material in a dual directional solidification furnace (topheater not shown). Side heater 132 and side heater 134 are adjustedtogether to create a horizontal silicon solidification gradient andconcentrate impurities proximate side heater 132. Initially, at furnacetemperature 1500° C., all silicon in the crucible is molten. In step136, the furnace temperature is adjusted to 1450° C. and the moltensilicon at the bottom of the crucible begins to solidify (see FIG. 12for a side view of silicon solidification in a dual directionalsolidification furnace) while the molten silicon moves proximate the topheater.

In step 138, at furnace temperature 1420° C., side heater 132 is heatedand side heater 134 is cooled—creating a horizontal siliconsolidification gradient. As the silicon proximate side heater 134 coolsand solidifies, the molten silicon moves proximate side heater 132.Impurities are gathered in the molten silicon proximate side heater 132.As the furnace temperature is reduced to 1400° C. in step 140, theremaining molten silicon, with concentrated impurity levels, solidifiesand impurities are captured in the ingot area proximate side heater 132.

FIG. 14 is a graphical depiction of a 3-D solidification interface of asilicon ingot created in a dual directional solidification furnace.Shown, the solid-liquid interface remained substantially planar duringingot crystallization resulting in substantially even and planarsolidification layers. Thus, the impurity profiles from top to bottomare substantially the same for any region of the silicon ingot.Solidification layers 150, 152, and 154 are planar throughout the ingot,unlike layers 90, 92, and 94 in FIG. 7. Further, as shown from the topview, the contaminated solidification layers have been furtherconcentrated on side 156 through the use of a dual directionalsolidification furnace, such as that shown in FIG. 13. This formationpermits impurities to be concentrated in areas that may be easilycropped according to the disclosed process. The dual directionalsolidification furnace is preferably run using crucibles with arectangular, non-quadratic cross section, whereby the smaller crucibleside is facing the side heater.

FIG. 15 is a graph showing the resistivity profiles (graphed as ohm-cmover solidified fraction g) and cropping lines for multipleconcentrations of impurities. The resistivity profile is stronglydependent on the impurity concentrations. This allows a determination ofthe impurity concentration at each point on the resistivity profile.Crop lines 166, 168, and 170 are dependent on the resistivity profile ofthe ingot. The crop lines may be determined based on the thresholdsilicon impurity concentrations allowed for the final product.

Ingot resistivity profile 160 has a boron concentration of 0.45 ppmw, aphosphorous concentration of 1.59 ppmw, and an aluminum concentration of0.087 ppmw. Crop line 166 corresponds to resistivity profile 160 and isthe controlled cut line yielding the correct impurity concentrationthreshold amounts for resistivity profile 160.

Ingot resistivity profile 162 has a boron concentration of 0.45 ppmw, aphosphorous concentration of 1.45 ppmw, and an aluminum concentration of0.079 ppmw. Crop line 168 corresponds to resistivity profile 162 and isthe controlled cut line yielding the correct impurity concentrationthreshold amounts for resistivity profile 162.

Ingot resistivity profile 164 has a boron concentration of 0.45 ppmw, aphosphorous concentration of 1.59 ppmw, and an aluminum concentration of0.119 ppmw. Crop line 170 corresponds to resistivity profile 164 and isthe controlled cut line yielding the correct impurity concentrationthreshold amounts for resistivity profile 164.

FIGS. 16-18 are graphs showing the relationship between an ingot'sresistivity profile and that same ingot's impurity concentrationprofile. A controlled crop line may be calculated depending on thedesired threshold concentration of a particular impurity. FIGS. 16-18show the crop line based on an aluminum concentration of 0.5 ppmw,however the crop line may be based on a number of various impurities(such as boron or phosphorous) at any concentration.

FIG. 16 illustrates the calculation of a cropping line from aresistivity profile and an impurity concentration profile for the samesilicon ingot. The top graph shows resistivity profile 182 (in ohm-cmover solidified fraction percentage) for a silicon ingot having a boronconcentration of 0.45 ppmw, a phosphorous concentration of 1.45 ppmw,and an aluminum concentration of 0.079 ppmw. The bottom graph shows aconcentration profile (in atoms per centimeter cubed over solidifiedfraction percentage) of boron 186, phosphorous 184, and aluminum 188 forthe same ingot. Crop line 180 has been calculated at an ingot height of84.5% for an aluminum concentration of 0.5 ppmw. Meaning the ingot belowcrop line 180 has an aluminum concentration of lower than 0.5 ppmw andthe ingot above crop line 180 has an aluminum concentration higher than0.5 ppmw.

FIG. 17 illustrates the calculation of a cropping line from aresistivity profile and an impurity concentration profile for the samesilicon ingot. The top graph shows resistivity profile 202 (in ohm-cmover solidified fraction percentage) for a silicon ingot having a boronconcentration of 0.45 ppmw, a phosphorous concentration of 1.45 ppmw,and an aluminum concentration of 0.117 ppmw. The bottom graph shows aconcentration profile (in atoms per centimeter cubed over solidifiedfraction percentage) of boron 208, phosphorous 204, and aluminum 206 forthe same ingot. Crop line 200 has been calculated at an ingot height of77% for an aluminum concentration of 0.5 ppmw. Meaning the ingot belowcrop line 200 has an aluminum concentration of lower than 0.5 ppmw andthe ingot above crop line 200 has an aluminum concentration higher than0.5 ppmw.

FIG. 18 illustrates the calculation of a cropping line from aresistivity profile and an impurity concentration profile for the samesilicon ingot. The top graph shows resistivity profile 224 (in ohm-cmover solidified fraction percentage) for a silicon ingot having a boronconcentration of 0.45 ppmw, a phosphorous concentration of 1.8 ppmw, andan aluminum concentration of 0.079 ppmw. The bottom graph shows aconcentration profile (in atoms per centimeter cubed over solidifiedfraction percentage) of boron 228, phosphorous 226, and aluminum 230 forthe same ingot. Crop line 222 has been calculated at an ingot height of84.5% for an aluminum concentration of 0.5 ppmw. Meaning the ingot belowcrop line 222 has an aluminum concentration of lower than 0.5 ppmw andthe ingot above crop line 222 has an aluminum concentration higher than0.5 ppmw. Crop line 220 has also been calculated at an ingot height of83% from the resistivity profile at the P/N change over—where the ingotmoves from p-type to n-type. This crop line reflects optimal cut line topreserve the highest yield of p-type silicon material from the ingot.

FIG. 19 is a graph showing the resistivity profiles (in ohm-cm oversolidified fraction percentage) of the silicon ingots in FIGS. 16-18.Resistivity profile 182 shows the resistivity of the ingot in FIG. 16and calculated crop line 180 at an ingot height of 84.5% for an aluminumconcentration of 0.5 ppmw. Resistivity profile 102 shows the resistivityof the ingot in FIG. 17 and calculated crop line 200 at an ingot heightof 77% for an aluminum concentration of 0.5 ppmw. Resistivity profile224 shows the resistivity of the ingot in FIG. 18 and calculated cropline 220 at an ingot height of 83.5% at the P/N changeover.

FIG. 20 presents the corresponding concentration profiles of boron,phosphorous, and aluminum for resistivity profiles 182, 202, and 224 inFIG. 19.

FIGS. 21 through 33 are directed towards a control process and methodfor evaluating UMG-Si feedstock quality. By analyzing the resistivityprofiles of crystallized ingot test samples made from multiple UMG-Sifeedstock batches, the boron and phosphorous content of those batchesmay be determined (and thus a determination of the quality of the UMG-Sifeedstock may be made). Further, other impurities such as, but notlimited to, SiC inclusions may also be detected.

FIG. 21 is a graphic depiction showing a prior art process flow forreducing boron, phosphorous, and aluminum content in silicon accordingto an inductively coupled plasma mass spectrometry (ICPMS) process. Instep 210, pure raw materials, such as quartz and coal, are selected toproduce MG-Si with low boron content. Step 212 then reduces the aluminumcontent further through MG-Si refinement. Additionally, boron contentcould be reduced further, for example in a furnace with an oxygen fuelburner—leading ultimately to UMG-Si. Then to further reduce impuritiessuch as boron, phosphorous, and aluminum, a chemical analysis of theUMG-Si is performed applying ICPMS (shown as step 214). If the analysisgives a boron concentration less than a specified thresholdconcentration (shown as 1 ppmw), the feedstock is regarded as ready forcrystallization and will be shipped for casting ingots, shown as finalUMG-Si product 216. However, if the boron concentration is measured tobe greater than the specified threshold concentration (shown as 1 ppmw),then the refinement process may be repeated until the material is aproper UMG-Si product meeting the minimum boron threshold concentrationlevels. Importantly, other threshold concentration levels for boron maybe used of other impurities, such as phosphorous.

More effective impurity controls are necessary to provide purer siliconwhile minimizing waste. The disclosed subject matter provides for analternative to the chemical analysis (ICPMS) described above and,instead, introduces another process and method for controlling UMG-Sifeedstock quality. The disclosed control method analyzes the resistivityprofiles of test ingots of the UMG-Si before the feedstock is released.This control method uses electrical data of reasonably large test ingotsmade from feedstock batches to be controlled. Specifically, measurementsof resistivity profiles from bottom to top of test ingots are criteriaof releasing UMG-Si feedstock batches as product.

As part of the disclosed process, a method for testing a plurality oftest ingots at the same time is provided, using a specially designedcrystal grower with a hot zone that can include top and bottom heatersor only a top heater such as the dual directional solidification furnaceshown in FIGS. 12 and 13. Thus, crystal growers having an N by N numberof crucibles may grow and test N×N test ingots. This procedure furtherimproves the method for controlling feedstock quality.

FIGS. 22 and 23 are graphs showing actual measured resistivity ofvarious batches of UMG-Si feedstock. Note the large UMG-Si feedstockvariability from the batch in FIG. 22 to the batch in FIG. 23 ofresistivity—and thus yield. The graphs of FIGS. 22 and 23 show theresistivity profiles (in ohm-cm over ingot height from bottom to top) oftwo ingots grown from two batches out of the same feedstock. Note thatthe resistivity profile reflects the change in conductivity type causedby the segregation characteristics of boron and phosphorous in theingot. The 150 mm tall ingot in FIG. 22 has a P/N change over—the pointwhere the ingot moves from p-type to n-type—at about 75 mm, leavingabout a 45% yield of p-type UMG-Si (shown as reference numeral 218) foruse. Table 219 provides resistivity data for the batch depicted in theresistivity profile of FIG. 22 including mean, median, minimum andmaximum resistivity values in ohm-cm. The 150 mm tall ingot in FIG. 23has a P/N change over at about 110 mm, leaving about a 74% yield ofp-type UMG-Si (shown as reference numeral 220) for use. Table 221provides resistivity data for the batch depicted in the resistivityprofile of FIG. 23 including mean, median, minimum and maximumresistivity values in ohm-cm.

This large variability originates mainly from incoming materials suchas, but not limited to, quartz and coal. The present disclosure proposesa method and process to control this variability before using suchfeedstock for casting industrial-size ingots, which are then used formaking solar cells after wafering.

FIG. 24 displays the ICPMS data of B (boron) and P (phosphorous) forexemplary test ingots of a simultaneous directional solidification runon four different batches—Batch 1, Batch 2, Batch 3, and Batch 4—in afour crucible run. Table 224 shows the measured boron and phosphorousconcentrations for Batch 1 and Batch 2. Table 226 shows the measuredboron and phosphorous concentrations for Batch 3 and Batch 4. Thecorresponding resistivity profiles are shown in FIG. 25 for Batch 1,FIG. 26 for Batch 2, FIG. 27 for Batch 3, and FIG. 28 for Batch 4. Here,the resistivity data are not in agreement with ICPMS based expectations.For example, one would expect similar resistivity profiles for Batch 1(as shown in FIG. 25) and Batch 3 (as shown in FIG. 27) based on themeasured values of boron and phosphorous. The measured resistivityprofiles for each batch allow for a realistic evaluation of thefeedstock quality—and not a chemical analysis.

At the same time, the amount of potential co-dopants may also bedetermined based on resistivity profiles in order to modify individualfeedstock batches—thus ensuring high p-type yields and a usefulresistivity range after co-doping.

FIG. 25 is a graph showing the measured resistivity data (resistivityprofile 230) for Batch 1 in FIG. 24 in resist in ohm-cm over ingotheight from bottom to top. Batch 1 P/N change over at about 120 mm,leaving about a 73% yield of UMG-Si (shown as reference numeral 234).Table 232 provides resistivity data for Batch 1 including mean, median,minimum and maximum resistivity values in ohm-cm.

FIG. 26 is a graph showing the measured resistivity data (resistivityprofile 236) for Batch 2 in FIG. 24 in resist in ohm-cm over ingotheight from bottom to top. Batch 2 P/N change over at about 45 mm,leaving about a 26% yield of UMG-Si (shown as reference numeral 240).Table 238 provides resistivity data for Batch 2 including mean, median,minimum and maximum resistivity values in ohm-cm.

FIG. 27 is a graph showing the measured resistivity data (resistivityprofile 240) for Batch 3 in FIG. 24 in resist in ohm-cm over ingotheight from bottom to top. Batch 3 P/N change over at about 50 mm,leaving about a 28% yield of UMG-Si (shown as reference numeral 246).Table 242 provides resistivity data for Batch 3 including mean, median,minimum and maximum resistivity values in ohm-cm.

FIG. 28 is a graph showing the measured resistivity data (resistivityprofile 248) for Batch 4 in FIG. 24 in resist in ohm-cm over ingotheight from bottom to top. Batch 4 P/N change over at about 70 mm,leaving about a 41% yield of UMG-Si (shown as reference numeral 252).Table 250 provides resistivity data for Batch 4 including mean, median,minimum and maximum resistivity values in ohm-cm.

FIG. 29 is a photograph of a casted ingot from bottom to top. Thepresent disclosure describes a method for controlling the quality ofUMG-Si feedstock by producing a small test ingot from every batch ofsilicon feedstock followed by measuring the resistivity from bottom totop. This process enables determination of growth conditions for ingots.One exemplary embodiment produces 450 kg ingots and specifies growthconditions for increasing p-type yield and resistivity control. However,present disclosure enables other such growth conditions.

FIG. 30 pictorially depicts embodiments of crystal grower crucibleconfigurations in accordance with the disclosed subject matter. Tocontrol feedstock quality, the present disclosure determines theconcentration of boron and phosphorous of feedstock materials from thebottom to top resistivity profiles of simultaneously grown (according toa directional solidification process) test ingots, using multi-cruciblecrystal growers that may adapt various setups, such as those shown inFIG. 30. Crystal grower formation 262 has a 2×2 crucible formation thatmay grow up to 4 test ingots during the same run. Crystal growerformation 264 has a 3×3 crucible formation that may grow up to 9 testingots during the same run. Crystal grower formation 266 has a 4×4crucible formation that may grow up to 16 test ingots during the samerun. Crystal grower formation 268 has a 6×6 crucible formation that maygrow up to 36 test ingots during the same run. Other crucible formationsmay also be used such as larger configurations (such as 7×7) or oblongrectangular formations (such as 2×3, 3×2, 3×4, or 4×3) or any variationthereof.

In an exemplary embodiment, the test ingots may weigh in the range of 15kg and the process grows these test ingots during the same run fromdifferent feedstock batches. Experimental tests confirm validcharacterization of the entire feedstock batch with this method.Typically, feedstock batches may range from 2000 kg to 6000 kg.

Further, the determination and controlling of boron and phosphorous infeedstock materials may be supplemented by detection of SiCcontamination. This may be done by detecting “inclusions” in ingots fromsuch feedstock materials using IR inspection.

The crucibles including the batches of feedstock materials in FIG. 30may be covered by lids made from high purity graphite or other suchmaterials, in order to avoid cross contamination during crystallization.The configurations shown in FIG. 30 are examples of possibleconfigurations for casting and testing different feedstock batchessimultaneously. In another embodiment, other configurations may usedifferent crucible shapes such as cylindrical shape.

FIG. 31 is a photographic representation showing a modification a singlecrucible thermal configuration to four (4) crucibles thermalconfiguration per directional solidification run. Single cruciblecrystal grower 270 has been modified to four crucible crystal grower272. Thus, four test ingots per directional solidification run may begrown simultaneously (as shown by the picture depicted in referencenumeral 274).

In this embodiment, individual ingot size allows for production of six(6) inch solar cells. The method of the present disclosure allows quickand reliable control of B/P ratio. The embodiment shown in FIG. 31 maybe scaled, for example, to thirty-six (36) ingots per run correspondingto a feedstock batch size of 50 MT of UMG-Si feedstock material.

The relatively small size of those test ingots allows for wellcontrolled crystallization using industrial scale crystallizationfurnaces with specially designed components for assuring thermal and gasflow symmetry. One ingot test per feedstock batch for B/P ratioconfirmation is used, and further analysis such as detection of SiCinclusions may follow.

FIG. 32 is photographic examples showing silicon carbide (SiC)impurities found in actual UMG-Si ingots. Note the varying degree of SiCinclusions in ingot 276, ingot 278, and ingot 280. SiC inclusions may bedetermined through an infrared imaging (IR) process. Often, depending onthe process conditions at a feedstock supplier's site, SiC inclusionsmay be formed from one batch to the other. Since feedstock with SiCinclusions produces ingots with inclusions, better control processes ata feedstock supplier's site allows production of inclusion-free ingotsat the user's (such as a solar cell manufacturer) site. In oneembodiment, the multi-crucible setup allows reliable process control offeedstock materials. Multi-crucible casting based control methodology isalso applicable to characterize incoming feedstock material forfeedstock users.

FIG. 33 is a process flow showing major steps of one embodiment of thedisclosed UMG-Si control process. In step 290, the feedstock materialbatches are selected for analysis. Typically, the batch size ranges fromapproximately two (2) to six (6) MT. For acceptable testing, theanalysis batch size vs. feedstock batch size should be greater than2×10⁻³. This ratio is 3-4 orders of magnitude greater than currentpractice based on chemical analysis of UMG-Si feedstock.

In step 292, crystallization of the test batches occurs. The size andshape of the crucibles typically allows for ingots capable of yieldingwafers on the order of 156 mm×156 mm. Further, since the disclosedprocess analyzes a plurality of batches in a single run, symmetricalthermal and gas flow conditions for all crucibles and test batchesshould be present.

In step 294, resistivity profiles of the individual ingots are measured.From this resistivity measurement, the concentrations of boron andphosphorous may be determined.

Optional step 296 determines co-dopants which may be used to increaseingot yield and produce appropriate resistivity profiles based on theamount of boron and phosphorous in the feedstock batches analyzed.Optional step 298 determines SiC inclusions in the test ingots throughIR analysis. And optional step 300 produces test wafers for extensiveassessment of the feedstock batches.

In operation, the disclosed subject matter provides a quality controlmethod to determine the concentrations of impurities in an UMG-Sifeedstock batch based on the resistivity profile of a test ingot madeUMG-Si from the batch. Multiple test ingots, each corresponding to aUMG-Si feedstock batch, may be grown simultaneously according to adirectional solidification process.

Although the disclosed subject matter has been described in detail, itshould be understood that various changes, substitutions, andalterations can be made hereto without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A control method for evaluating the quality of aUMG-Si feedstock, the method comprising the steps of: performing adirectional solidification of molten upgraded metallurgical-gradesilicon (UMG-Si) from a UMG-Si feedstock batch to form a silicon testingot; measuring the resistivity from top to bottom of said silicon testingot; mapping the resistivity profile of said silicon test ingot;calculating the phosphorus and boron concentrations of said UMG-Sifeedstock batch based on said resistivity profile of said silicon testingot; and determining, based on the resistivity profile of said silicontest ingot, an amount of at least one co-dopant for use in said UMG-Sifeedstock batch for increasing an ingot yield and producing an improvedresistivity profile by directional solidification.
 2. The method ofclaim 1, wherein said step of calculating the phosphorus and boronconcentrations of said selected UMG-Si feedstock batch based on saidresistivity profile of said silicon test ingot further comprises thestep of calculating the phosphorus and boron concentrations of saidselected UMG-Si feedstock batch based on the yield of said silicon testingot determined from said resistivity profile of said silicon testingot.
 3. The method of claim 1, further comprising the step ofdetermining SiC inclusions in said silicon test ingot through IR imaginganalysis.
 4. The method of claim 1, further comprising the step ofproducing test wafers from said silicon test ingot.
 5. The method ofclaim 1, wherein the weight ratio of said silicon test ingot to saidUMG-Si feedstock batch is greater than 2×10⁻³.
 6. The method of claim 1,wherein said silicon test ingot weighs approximately 15 kg.
 7. Themethod of claim 1, wherein said step of performing a directionalsolidification uses a dual directional solidification furnace thatconcentrates impurities on the top and one side of said silicon testingot.
 8. A control method for evaluating the quality of UMG-Sifeedstock, the method comprising the steps of: performing a simultaneousdirectional solidification of molten UMG-Si from a plurality of UMG-Sifeedstock batches in a single crystal grower to form a plurality ofsilicon test ingots, wherein each of said plurality of silicon testingots corresponds to a particular UMG-Si feedstock batch; measuring theresistivity from top to bottom of each of said silicon test ingots;mapping the resistivity profile of each of said silicon test ingots;calculating the phosphorus and boron concentrations of each of saidUMG-Si feedstock batches based on said resistivity profile of each ofsaid corresponding silicon test ingot; and determining, based on theresistivity profiles of said silicon test ingots, an amount of at leastone co-dopant for use in said plurality of UMG-Si feedstock batches forincreasing an ingot yield and producing an improved resistivity profileby directional solidification.
 9. The method of claim 8, wherein saidstep of performing a simultaneous directional solidification of moltenUMG-Si from a plurality of UMG-Si feedstock batches in a single crystalgrower to form a plurality of silicon test ingots, wherein each of saidplurality of silicon test ingots corresponds to a particular UMG-Sifeedstock batch further comprises performing a simultaneous directionalsolidification of molten UMG-Si from a plurality of UMG-Si feedstockbatches in a single multi-crucible crystal grower to form a plurality ofsilicon test ingots, wherein each of said plurality of silicon testingots corresponds to particular UMG-Si feedstock batch.
 10. The methodof claim 8, wherein said step of calculating the phosphorus and boronconcentrations of each of said UMG-Si feedstock batches based on saidresistivity profile of each of said corresponding silicon test ingotsfurther comprises the step of calculating the phosphorus and boronconcentrations of each of said selected UMG-Si feedstock batches basedon the yield of each of said silicon test ingots determined from saidresistivity profile of each of said silicon test ingots.
 11. The methodof claim 8, further comprising the step of determining SiC inclusions insaid silicon test ingot through IR imaging analysis.
 12. The method ofclaim 8, further comprising the step of producing test wafers from eachof said silicon test ingots.
 13. The method of claim 8, wherein theweight ratio of each of said silicon test ingots to each of saidcorresponding UMG-Si feedstock batches is greater than 2×10⁻³.
 14. Themethod of claim 8, wherein each of said silicon test ingots weighsapproximately 15 kg.
 15. The method of claim 8, wherein said step ofperforming a directional solidification uses a dual directionalsolidification furnace that concentrates impurities on the top and oneside of each of said silicon test ingots.
 16. The method of claim 8,wherein said step of performing a simultaneous directionalsolidification of molten UMG-Si from a plurality of UMG-Si feedstockbatches in a single crystal grower to form a plurality of silicon testingots, wherein each of said plurality of silicon test ingotscorresponds to particular UMG-Si feedstock batch further comprisesperforming a simultaneous directional solidification of molten UMG-Sifrom a plurality of UMG-Si feedstock batches in a single multi-cruciblecrystal grower having an N×N crucible formation to form a plurality ofsilicon test ingots, wherein each of said plurality of silicon testingots corresponds to a particular UMG-Si feedstock batch.